Intro to Ultrasonic Phased Array
Many people are familiar with the medical applications of ultrasonic imaging, in which high frequency sound waves are used to create highly detailed cross-sectional pictures of internal organs.
Medical sonograms are commonly made with specialized multi-element transducers known as phased arrays and their accompanying hardware and software. But the applications of ultrasonic phased array technology are not limited to medical diagnosis.
In recent years, phased array systems have seen increasing use in industrial settings to provide new levels of information and visualization in common ultrasonic tests that include weld inspection, bond testing, thickness profiling, and in-service crack detection.
What is a phased array system?
Conventional ultrasonic transducers for NDT commonly consist of either a single active element
that both generates and receives high frequency sound waves, or two paired elements, one for transmitting and one for receiving.
Phased array probes, on the other hand, typically consist of a transducer assembly with from 16 to as many as 256 small individual elements that can each be pulsed separately.
These may be arranged in a strip (linear array), a ring (annular array), a circular matrix (circular array), or a more complex shape.
Phased array probes may be designed for direct contact use, as part of an angle beam assembly with a wedge, or for immersion use with sound coupling through a water path.
Transducer frequencies are most commonly in the range from 2 MHz to 10 MHz. A phased array system will also include a sophisticated computer-based instrument that is capable of driving the multi-element probe, receiving and digitizing the returning echoes, and plotting that echo information in various standard formats.
Unlike conventional flaw detectors, phased array systems can sweep a sound beam through a range of refracted angles or along a linear path, or dynamically focus at a number of different depths, thus increasing both flexibility and capability in inspection setups.
PHASED ARRAY UT vs Automated UT (AUT)
1. AUT reconstructs the test piece cross-section (B-scan) after taking data using a single refracted angle and scanning it back and forth on the test piece. PAUT reconstructs such image from a single probe location with no scanning.
2. In many cases, especially for thin plates, a single line scan will perform the inspection. AUT always requires either a 2-axis scan or multiple probes to reconstruct the image. Line scan done with PAUT scanning is much simpler than raster scanning.
3. On applications that require 16 to 32 probes with AUT, PAUT can be done with significantly lesser number of probes, eg. one array on either side of the weld.
4. PAUT requires significantly less inspection space for scanning compared to AUT.
5. Both PAUT and AUT store raw A-scan data that can be replayed for analysis.
PHASED ARRAY UT vs MANUAL UT
1. Manual UT produces a single A-scan at a specific angle. Manual UT evaluation requires plotting the indication using the refracted angle, metal path and surface distance. PAUT displays images in real time showing the depth and location of indication relative to the probe.
2. Manual UT is limited to a single refracted angle. PAUT simultaneously takes data from a range of angles, eg 40 to 75 degrees and reconstructs an image in real time.
3. PAUT image is easy to comprehend as it gives a display of the ultrasound superimposed on the test piece.
4. Using an encoder with the PAUT probe, all raw A-scan data can be stored. Once stored, the data can be replayed. This is most important to retain a complete record of the inspection. There is no data storage capability in manual UT.